Ornithodoros concanensis and possibly Argas cooleyi orient to their hosts over distances as great as 5 to 10 m by responding to host's vocalizations. By sonographic analysis of host sounds and the testing of various sound frequencies and patterns of modulation, it should be possible to determine the elements of the host vocalizations which are most effective in stimulating host-finding activity among unfed ticks. A tick's response to sound seems to depend upon its conditioning prior to exposure to the sounds. The effects of factors such as the daily light cycle, age, nutritional state, extent and circumstances of prior exposure to sound, and exposure to elevated levels of CO2 and host odors will be tested. The mechanics of the orientation process will be studied and an effort will be made to locate and describe the morphology of the sound receptors.